The invention relates to a colour display tube system comprising
a) an evacuated envelope having a neck, a cone and a display window,
b) an electron gun in the neck, which gun has a bean-forming part for generating a central electron beam and two outer electron beams whole axes are co-planar, and a first and a second electrode system which in operation jointly constitute a main lens and are connectable to means for supplying an energizing voltage, and
c) a deflection unit for generating deflection fields for deflecting the electron beams in the horizontal and vertical directions and for scanning the display window by means of convergent beams.
Colour display tube systems of the type described in the opening paragraph are of the conventional 3-in-line type. They generally comprise self-convergent deflection units which in operation generate non-uniform magnetic fields for horizontal and vertical deflection (particularly a barrel-shaped field for the vertical deflection and a pincushion-shaped field for the horizontal deflection) so that the three electron beams generated by the electron gun and focused on the display screen by the main lens converge throughout the display window.
However, these self-convergent fields cause the horizontal spot growth to increase by a given factor in the case of deflection, which factor may be more than two in 110.degree. colour display tube systems. This notably means that in a normal self-convergent system, in which the three guns are located in a horizontal plane, the circular central spot becomes flat in the vertical direction and very elongate in the horizontal direction, particularly when using a gun having a dynamic astigmatic focusing facility and when scanning the screen. As a result loss of resolution occurs in the horizontal direction and there is a risk of Moire problems owing to the spot becoming flatter and the existence of horizontal dams in the shadow mask. The increasingly stricter requirements imposed on the homogeneity and the definition of the image, notably in high-resolution colour monitor tubes or when using colour display tubes for high-definition television with a display screen having a 9:16 aspect ratio imply that the spot throughout the screen should be as small and as uniform as possible.